An endorsement is one of the required components on a researcher's project page before they can launch. Each endorser should either be an expert in your field of study, or they should be able to speak to your qualifications as a researcher. You can read more about requesting endorsements and writing endorsements (for your endorser) in our researcher guide.

Backing a project simply means contributing funds to a project on Experiment, which cannot be done until after the project launches or is "live."